Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

Electron beam interactions with CO on W {100} studied by Auger electron spectroscopy

The interaction of 2500 eV electrons with carbon monoxide chemisorbed on tungsten {100} was investigated by rapid-scan Auger electron spectroscopy. When no α state was present the O and C signals from the β state of CO were invariant during electron bombardment, giving an upper limit estimate for the electron stimulated desorption cross section, Q_β of 2 × 10^?21 cm². With the crystal at room temperature and saturated with CO, however, electron-beam induced accumulation of carbon was observed and characterised, the rate of the process being independent of CO pressure at pressures above 2 × 10^?8 Torr. At 450 K the rate was found to be pressure dependent up to at least 6 × 10^?7 Torr. A model is proposed for the accumulation process, which is based on electron beam dissociation of α₂-CO to form adsorbed carbon and gaseous O and the creation of new sites for further α₂-CO adsorption; it is in quantitative agreement with the results and yields a cross section for ESD of α₂-CO (Q_α2 = 1.55×10^?18cm²) in close agreement with direct measurements.     

Double charge transfer in H+ + Mg Collisions

The total cross section for the reaction H⁺ + Mg → H^? + Mg⁺⁺ is calculated in the energy range of 10-10 000 eV. The double charge exchange cross section is large, rising to a maximum of 4× 10^?16 cm² at 100 eV.     

Relaxation des Natrium-Grundzustandes bei Stößen mit den stabilen Helium- und Wasserstoff-Isotopen

The spin-polarization of the optically pumped Na-groundstate is investigated in presence of the stable He- and H₂-isotopes. The following disorientation cross sections are derived from the pressure dependence of the relaxation rate:σ=(16.2±2.0) · 10^?26 cm² for He³ σ=(2.4±0.5) · 10^?26 cm² for He⁴ σ=(3.9±1.5) · 10^?26 cm² for H₂ σ=(2.3±1.0) · 10^?26 cm² for D₂. These values can be compared with theoretical cross sections based on two relaxation models and indicate the existence of a relaxation mechanism involving the exchange of the electronic alkali-spin with the nuclear spin of the foreign gas.     

Efficient ～2 μm emission and energy transfer mechanism of Ho3+ doped barium gallium germanate glass sensitized by Tm3+ ions

Optical absorption and emission properties of Ho³⁺ ions in barium?Cgallium-germanate glass are investigated. A significant result is that the emission intensity of Ho³⁺:⁵I₇??⁵I₈ excited by 808 nm through the energy level Tm³⁺:³H₄ is stronger than that of the direct excitation of Ho³⁺. The emission cross section of Ho³⁺ is calculated to be 9.02×10^?21?cm² at the wavelength of 1.994 ??m. Comparatively, the larger emission cross section and efficient energy transfer process reveal that the prepared glass could be a kind of promising material for laser emission at 2 ??m.     

Experimental cross section for dimuon production and the Drell-Yan model

We have measured the absolute cross section for dimuon production in hadron collisions at 200 GeV/c in the continuum region M = 4–8.5 GeV. In all the channels studied (pN, $\text{p}$N, π^±N and π^?H₂) the experimental cross section is significantly larger by a factor of 2.3 ± 0.5 than expected from the Drell-Yan model. Furthermore, our proton-nucleon data allow a determination of the nucleon valence structure function which agrees with the deep inelastic lepton scattering data.     

On the origin of electron stimulated desorption of H+ from metal surface

The emission of hydrogen ESD ions from polycrystalline Nb is studied experimentally. The results demonstrate clearly that no ESD emission of H⁺ occurs from a pure H adsorbate. However, adsorption of H₂O, present as an impurity in H₂, and H₂ adsorption on Nb preceded by O adsorption, produce strong H⁺ ESD emission. It is shown that Auger ionization of O can account for the increased H⁺ yield and it is postulated that similar effects also occur on other metal surfaces.     

Schwingungsanregung von H2-Molekülen durch Zentralstoß mit Li+-Ionen

An apparatus is described for measuring the inelastic differential cross section for vibrational excitation in collisions of diatomic molecules with monoenergetic ions at laboratory energies between 10 and 50 eV. The method consists of measuring the time of flight of single ions with a time amplitude converter and displaying the results on a 100 channel pulse height analyzer. From the shift in the time of flight relative to that expected for elastic scattering the final state of the molecule excited in a single collision is identified. By studying only central collisions with almost zero impact parameter rotational excitation is strongly suppressed. Measured times of flight after collisions of monoenergetic Li⁺ ions with H₂ show that with increasing energy the most probable vibrational quantum jump increases from 0→1 to 0→2,0→3 etc. Contrary to the usual assumption of a small steric factor for vibrational excitation the results show that the inelastic cross section is larger than the elastic cross section. Using reported potential parameters the energy dependence of the most probable excited state is compared with the calculations of Secrest and Johnson for a one-dimensional collinear collision. The satisfactory agreement suggests that the steric factor is close to 1. From measurements at different scattering angles at 10 eV the integral inelastic cross section is found to be about 0.2 Ų corresponding to a differential cross section of 0.4 Ų/sr. Measured values of integral and differential total cross sections for Li⁺-He andLi⁺-H₂ are reported and compared with theory. Direct dissociation of D₂ by Li⁺ in the energy range from 25 to 55 eV was not observed, yielding an upper limit for the cross section of 4 · 10^?4 Ų/sr.     

氢负离子在微腔中的光剥离研究

利用闭合轨道理论,研究了氢负离子在金属面和弹性界面组成的微腔中的光剥离截面.结果表明,微腔的上下表面对氢负离子的光剥离截面产生很大影响.若固定金属面与氢负离子之间的距离不变,当弹性界面与氢负离子之间的距离很大时,弹性界面对氢负离子光剥离截面的影响很小,光剥离截面的振荡幅度和振荡频率与只有金属面时的情况类似.随着弹性界面和氢负离子之间距离的不断减小,光剥离截面的振荡幅度增大,振荡频率减小.若保持氢负离子和弹性界面之间的距离不变,随着金属面和氢负离子之间距离d₀的不断增大, 关键词： 光剥离截面 闭合轨道理论 微腔     

Adsorption of hydrogen and carbon monoxide on platinum

TDS spectra and ESD ion energy distributions have been measured for coadsorption of H₂ and CO on recrystallized platinum. Sample exposure to a 90% H₂?10% CO gas mixture results in appearance of structurein both TDS spectra of H₂ and CO. Coadsorption also results in an O⁺ ion energy distribution which is much narrower compared to the distribution resulting from pure CO adsorption. These results are interpreted as evidence for the formation of an HCO surface complex.     

Quantum rotational scattering of H2 and its isotopologues with He

Quantum close-coupling computations of the rotational quenching of H₂ and its isotopologues due to He impact are performed using a highly accurate potential energy surface. State-to-state cross sections are obtained in a wide range of collision energies between 10^?5 cm^?1 and 10⁴ cm^?1, and the theoretical rate coefficients are reported for temperatures ranging from 10^?4 K to 3000 K. Compared with previous studies, the well depth of the potential adopted in this study is larger, leading to stronger resonance effects in the cross sections of He-HD. The accurate potential was employed to investigate the isotope effects of H₂ in detail. The cross-section resonances shift towards lower collision energies and become stronger with increasing reduced mass. The calculated cross sections and rate coefficients of H₂ and its isotopic variants in collisions with He are provided to study the energy transfer in these systems.     

Autoionization cross section of potassium atom excited by electron impact

The autoionization cross section of potassium atoms excited by electron impact is measured in the energy range from the first autoionization threshold at 18.72 eV to 202 eV. The data are obtained by deter-mining the total intensity of electron spectra resulting from the decay of the 3p ⁵ n ₁ l ₁ n ₂ l ₂ autoionizing states. The cross section has two maxima, 1.8 × 10^?16 and 2.2 × 10^?16 cm², at 21 and 32 eV, respectively. The excitation dynamics of autoionization states suggests that the first maximum is associated with the resonance character of the near-threshold excitation. The second maximum, as well as the behavior of the cross section at energies above 50 eV, reflects the dynamics of electron excitation of quartet and doublet autoionizing states. The measured autoionization cross section is compared with known data for the total single ionization cross section of potassium atom by electron impact. The relative contribution of the autoionization cross section to the total single ionization cross section is found to reach 30% at 32 eV.     

Ladungsaustausch-Reaktionen einiger negativer Ionen mit O2 und die Elektronenaffinität des O2

Investigations on the energy dependence of the cross section of charge transfer reactions with negative ions yield evidence on the relative values of the electron affinities of the colliding particles. It is shown that H^?+ O₂ and SO^? + O₂ are exothermic and that ND ₂ ^? + O₂ is anendothermic reaction. With EA(H)=0.776eV, EA(SO)=1.05eV and EA (ND₂) ≈ 1.2 eV we find 1.05 eV < EA (O₂) < ≈ 1.2 eV.     

The holographic bubble chamber experiment and the determination of the effective charmed quark mass and theK-factor for hadronic charm production

In the first holographic bubble chamber experiment — the HOBC experiment — we have accumulated a total of 40000 holograms with particle interactions. We have determined the total charm pair cross section inpN collisions to be 23.3 _?7.7 ⁺¹⁰ μb and 3.6 _?1.7 ^+2.3 μb for 360 and 200 GeV/c incident protons respectively. We have assumed a linear dependence of the cross section on the atomic number of the target. This experiment has demonstrated the feasibility of holographic recording in small bubble chambers. Assuming that the charm cross section can be described by the standard QCD factorized expression with gluon fusion and quark-antiquark annihilation, we have used our measured charm cross sections with other measurements to determine the effective charmed quark mass to be 1.8 _?0.35 ^+0.25 GeV/c². TheK factor, which describes the importance of the higher order corrections, is calculated to be 9.8 _?6.9 ^+12.5 (See noted added in proof.)     

L α emission from collisional excitation of H Atoms by threshold energy Ar+ ions

In a specially designed collision chamber, H atoms produced by a microwave discharge were excited into the 2p state by impact of Ar⁺ ions in a beam. A degree of dissociation up toη=94% was measured at the exit of the microwave source, although in the center of the collision cellη is smaller, with a lower limit of 33 %. The effect of vibrational excitation of H₂ by the discharge and the resulting difficulties of measuringη are discussed. The cross section forL _α emission increases linearly from the threshold, at 10.2 eV_CM, to 17.1 eV_CM. The absolute cross section at this energy is 1.5·10^?17 cm² forη=33% (with a minimum of 0.6·10^?17 cm², for the maximum possibleη). The absence of an activation barrier rules out some features shown by an earlier ab initio calculation.     

Calculation of the cross sections for the photoionization of H2 and D2 into different vibrational states of the ion

The dependence of the electronic transition moment on the internuclear distance is explicitly taken into account in the calculation of the photoionization cross section. Thus, the partial ionization cross sections for producing different vibrational states of the residual ion are obtained without invoking the Franck-Condon factor approximation. The exact electrostatic potential of H₂⁺, as well as the two-center Coulomb field, is used in the evaluation of the continuum wave function. The result can explain fairly well the ratios of the partial cross sections measured at 584 Å. The effect of the polarization of H₂⁺ due to the departing photoelectron is also studied and found to be small as far as the relative cross section is concerned. The total photoionization cross sections of H₂ and D₂ are compared with previous results obtained by other authors.     

Possible resonance structure in the elastic scattering of neutrons by Y near En = 1 MeV

The total cross section as well as the differential cross section and polarization in the elastic scattering of 0.8–1.4 MeV neutrons by Y have been measured with neutron beams of energy spread less than 20 keV. Rather weak structure with widths ≈ 50 keV was observed at a few energies within this range. The data were analyzed by use of a model in which the scattering process is described in terms of resonance amplitudes superimposed on an optical-potential background. Although not completely definitive, this analysis indicates the existence of three intermediate-width resonances (two 1^? and one 1⁺) at neutron energies between ≈ 1.0 and 1.2 MeV. The properties of the 1^? resonances suggest that these are the parent states of the proposed T_> components of the El giant resonance observed near 21 MeV excitation energy in ⁹⁰Zr produced in the ⁸⁹Y(p,γ₀) reaction. The resolved resonance structure in this energy region is in reasonable agreement with a recent calculation of the energies and widths of negative-parity states in ⁹⁰Y.     

Influence of anomalous thresholds in electron-positron annihilation into hadrons: The e+e− → ϱππ cross section

We calculate the cross section for e⁺e^? → ?ππ on the basis of partial-wave dispersion relations in the ππ channel taking proper account of anomalous singularity contributions. The appearance of anomalous thresholds is due to the fact that the vertex γ_V?(ππ) becomes internally unstable as the virtual photon mass is increased. Reasonable agreement with existing data is found. The anomalous singularity contributions provide by far the dominant part of the cross section which is a warning to using naive vector dominance extrapolations in estimating the electron-positron cross section.     

Calculations of inelastic cross sections for rotational excitation

Cross sections for scattering of N₂ (j=0) molecules on He atoms have been calculated for relative energies below 5 · 10^?3 eV (58 °K). The time independent scattering formalism ofArthurs andDalgarno was used together with an assumed Lennard-Jones type potential with anisotropicP ₂(cos?) terms in the attractive and repulsive parts. The resulting system of coupled differential equations was solved in the distorted wave and close coupling approximations for the differential and integral cross sections for elastic and inelastic (j=0→j=2) scattering. In the integral inelastic cross section several sharp resonances were found to contribute 40% to the cross section at energies below 40 °K. The resonance peaks are attributed to orbiting or short-lived compound states since they are also observed in the elastic cross section at energies which are lower by the excitation energy of 1.5 · 10^?3 eV. Finally, the effect of varying the potential parameters on the integral inelastic cross section was studied at 50 °K and a rough formula for the cross section as a function of the parameters is obtained. The formula shows that a certain ratio of repulsive and attractive anisotropies leads to a small inelastic cross section indicating a mutual cancellation.     

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H₂ have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β₂, β₁, and β₀ were detected after adsorption at 250 K. The population of β₂ and β₁ states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O⁺ and CO⁺ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H⁺ signal was recorded following H₂ adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H⁺ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H₂ TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β₂ state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H⁺ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.